In accordance with well known physical principles, whenever a charged particle is placed in an environment wherein a magnetic field is crossed with an electric field (i.e. the magnetic field is perpendicular to the electric field), the charged particle will be forced to move in a direction that is perpendicular to the plane of the crossed fields. For configurations wherein the electric field is radially oriented perpendicular to a central axis, and the magnetic field is oriented parallel to the central axis, the charged particle will be forced to move along circular paths around the central axis. This circular motion, however, generates centrifugal forces on the charged particle that will cause the particle to also move outwardly and away from the central axis.
In addition to the phenomenon described above, it is also known that charged particles will tend to travel through a magnetic field in a direction that is generally parallel to the magnetic flux lines. Thus, for the situation described above wherein the magnetic flux lines are oriented substantially parallel to a central axis of rotation, the magnetic flux lines will generally oppose the centrifugal force that is exerted on a charged particle as the particle rotates about the axis of rotation. It happens, however, that this opposing force is generally proportional to the magnitude of the magnetic field, with a lower magnitude magnetic field giving less opposition to the movement of the particle than a higher magnitude magnetic field.
Because the magnitude of a centrifugal force acting on a charged particle is a function of the mass of the particle, it follows that, for a given condition (i.e. for given crossed electric and magnetic fields), high-mass particles will experience higher centrifugal forces than will low-mass particles. Indeed, plasma centrifuges which are used for the purpose of separating charged particles from each other according to their respective masses (e.g. multi-species plasmas) rely on this fact. Centrifuges, however, also rely on a condition wherein the density of the plasma in the centrifuge chamber is above its so-called "collisional density" and on the fact that the electric field is directed away from the axis of rotation. In comparison with a plasma centrifuge, for a condition wherein the density of the plasma is maintained below the "collisional density" and wherein the electric field is directed toward the axis of rotation, a much different result is obtained.
It can be mathematically shown that when using a cylindrical shaped chamber which has a wall that is located at a distance "a" from the central longitudinal axis of the chamber; with a magnetic field, B.sub.z, oriented in a direction substantially parallel to the longitudinal axis of the chamber; and with an electric field established with a positive potential "V.sub.ctr " on the longitudinal axis and a substantially zero potential on the wall, where "e" is the electric charge on the ion, an expression pertains wherein: M.sub.c =ea.sup.2 (B.sub.z).sup.2 /8V.sub.ctr. In this expression, M.sub.c is an effective cut-off mass which differentiates between high-mass particles and low-mass particles. For environments inside a plasma chamber wherein the mass of a multi-species plasma is maintained below its "collisional density," M.sub.c can be established such that the high-mass particles in a multi-species plasma (i.e. those particles which have a mass greater than the cut-off mass) will be ejected into the wall of the chamber as the plasma transits the chamber. Low-mass particles, on the other hand, will not be ejected during their transit of the chamber.
Recall that the movement of charged particles in a direction which is across or perpendicular to the magnetic flux lines will be generally opposed by the magnetic field. Further, this opposition will be generally proportion to the magnitude of the magnetic field. Like other magnetic field environments, this opposition also pertains to the specific situation for a plasma rotating around an axis and in an environment wherein the electric field is directed to extract ions resulting in a cut-off mass of M.sub.c =ea.sup.2 (B.sub.z).sup.2 /8V.sub.ctr. Thus, by decreasing the magnitude of the magnetic field near the central axis of rotation in a cylindrical shaped plasma chamber, there will be decreased resistance to the outwardly radial movement of rotating charged particles away from the central axis. At the same time, because low-mass charged particles will experience lower centrifugal forces than will the high-mass particles, the low-mass particles will react more slowly and, therefore, will be more likely to remain nearer the central axis. Consequently, these trends will facilitate the movement of high-mass charged particles away from the central axis and into the region of the plasma chamber where the expression, M.sub.c =ea.sup.2 (B.sub.z).sup.2 /8V.sub.ctr becomes more effectively operable. Importantly, with an increased efficacy in the separating of particles, there is also the ability to increase throughput.
In light of the above, it is an object of the present invention to provide a centrifugal mass filter which, for given crossed magnetic and electric fields, will facilitate the movement of both high-mass and low-mass charged particles into a region where they can be effectively separated from each other. It is another object of the present invention to provide a centrifugal mass filter which more predictably confines low-mass particles in the chamber, and more predictably ejects high-mass particles from the chamber, during their respective transit through the chamber. Yet another object of the present invention is to provide a centrifugal mass filter which will effectively process increased throughput. It is another object of the present invention to provide a centrifugal mass filter which is relatively easy to manufacture, is easy to operate and is comparatively cost effective.